The invention relates to an electrode for a plasma torch and a plasma torch head with said plasma torch.
A plasma is an electrically conductive gas consisting of positive and negative ions, electrons and excited and neutral atoms and molecules heated thermally to a high temperature. Various gases are used as plasma gases, such as mono-atomic argon and/or the diatomic gases hydrogen, nitrogen, oxygen or air. These gases are ionised and dissociated by the energy of an electric arc. An electric arc can be constricted by a nozzle and is then referred to as a plasma jet.
The parameters of a plasma jet can be heavily influenced by the design of a nozzle and electrode. Such parameters of a plasma jet include, for example, the diameter of a jet, temperature, energy density, and gas flow rate.
In plasma cutting, for example, plasma is constricted by a nozzle, which can be cooled by gas or water. In this way, energy densities of up to 2×106 W/cm2 can be achieved. Temperatures of up to 30,000° C. can arise in a plasma jet, which, in combination with the high flow rate of the gas, make it possible to achieve very high cutting speeds on materials.
Due to the high thermal stresses they encounter, nozzles used are usually made from a metallic material, preferably copper, to benefit from high electrical conductivity and thermal conductivity. The same is true of electrode holders, though electrode holders may also be made of silver. A nozzle can then be inserted into a plasma torch, the main elements of which are a plasma torch head, a nozzle cap, a plasma gas conducting member, a nozzle, a nozzle holder, an electrode quill, an electrode holder with an electrode insert and, in modern plasma burners, a holder for a nozzle protection cap and a nozzle protection cap. The electrode holder can fix a pointed electrode insert, known as an emission insert, made from tungsten, which is suitable when non-oxidizing gases, such as a mixture of argon and hydrogen, are used as the plasma gas. A flat-tip electrode, the electrode insert of which is made of hafnium, is also suitable when oxidizing gases, such as air or oxygen, are used as the plasma gas.
The nozzle and electrode are often cooled with a liquid, such as water, to achieve a extend service life. Such cooling can also be effected with a gas. This leads to a recognized distinction between liquid-cooled and gas-cooled plasma torches.
An electrode can comprise an electrode holder, which can be made from a material with good electric and thermal conductivity, e.g. copper and silver or their alloys, and an emission insert consisting of a temperature-resistant material, e.g. tungsten, zirconium, or hafnium. For plasma gases containing oxygen, zirconium can be used, though hafnium may be better suited due to superior thermal properties, since the oxide of hafnium is more temperature-resistant.
To improve electrode service life, a temperature-resistant material can be introduced into the holder as an emission insert, which is then cooled. Normally, the most effective form of cooling is liquid cooling.
Former East Germany document DD 87361 B1 describes an apparatus utilizing an electrode (cathode) for oxidising gas. A disclosed cathode (emission insert) consists of a material, e.g. zirconium, the oxide of which is temperature-resistant and which is inserted into a cathode holder made of copper. A disclosed cathode holder is cooled from the inside by a cooling water channel. DD 87361 B1 also describes the problem of limited endurance (short service life) of the cathode, caused by the rotation of the plasma gas, which is necessary for cut quality. The cathode holder has a collar with a gas conducting ring arranged around it, which has gas channels incorporated in it to divide the plasma gas into a partial stream and a main stream, forming the main stream on the side facing the nozzle and causing it to rotate while the partial stream on the side facing the cathode holder rotates in the opposite direction. Alternatively, the collar of the cathode holder may have recesses serving to form and deflect a partial gas stream. The intended effect is to create a calmed gas zone upstream of the emission insert to reduce its wear. However, such method results in cut quality that is inferior to those obtained utilizing powerfully rotating plasma gas.
German documents DE 690 14 289 T3 and DE 699 37 323 T2 disclose electrode arrangements in which a sleeve (separator) is attached around an emission insert, which separates the emission insert from an electrode holder. In such arrangement, the separator consists mainly of silver and the electrode holder mainly of copper. The utilized silver ensures long service life, especially when cutting with pure oxygen, because silver reacts more inertly with oxygen than does copper. However, such electrode arrangements have the significant disadvantage of being complex to manufacture.
German document DE 695 12 247 T2 discloses an apparatus in which the emission surface of an emission insert is initially shaped such that it determines a recess in the emission insert, having an initial depth in the central axis proportional to the cutting stream and the diameter of the emission insert. Such recess causes deposits of emission material on the inside surface of the nozzle resulting from the ignition and operation of the plasma arc to be reduced. Studies have shown, however, that such arrangements cannot actually extend apparatus service life.